BEIRUT  Rebels backed by captured tanks launched a fresh offensive on a government complex housing a police academy near the northern city of Aleppo on Sunday, while the government hit back with airstrikes to try to protect the strategic installation, activists said.

If rebels capture the complex on the outskirts of Aleppo, it would mark another setback for President Bashar Assad. In recent weeks, his regime has lost control of key infrastructure in the northeast including a hydroelectric dam, a major oil field and two army bases along the road linking Aleppo with the airport to its east.

Rebels also have been hitting the heart of Damascus with occasional mortars shells or bombings, posing a stiff challenge to the regime in its seat of power.

On Saturday, opposition fighters in the eastern province of Deir el-Zour overran a military post believed to have once been the site of a partly built nuclear reactor that Israeli warplanes bombed in 2007.

A year after the strike, the U.N. nuclear watchdog determined that the destroyed building’s size and structure fit specifications of a nuclear reactor. Syria never stated the purpose of the site known as Al-Kibar.

After the bombing, the regime carted away all the debris from the destroyed building and equipment from the two standing structures, analysts said, adding that the rebels were unlikely to have found any weapons in the abandoned complex.

There were troops in the area until this weekend. It was not clear what the site was being used for most recently.